Fiber reinforced polymer components, otherwise known as polymer composite components, consist of reinforcing fibers held together by a polymer resin, often known as the matrix. This matrix can be a thermosetting polymer such as an epoxy resin, in which case the composite component can be called a thermoset composite component, or a thermoplastic polymer such as polyamide or polyetheretherketone, in which case the component can be called a thermoplastic composite component. It should be noted that a thermoset composite component may contain small amounts of thermoplastic polymer, for instance as a surfacing film, a resin additive, or a binder agent. A thermoplastic composite component may in the same way contain small amounts of thermoset polymer, for instance in a core or insert.
Most large structural thermoset composite components such as carbon-epoxy components are made using open (one-sided) molds. A vacuum bag or other soft tooling surface is used to compact the reinforcing fibers and unreacted resin against the stiff mold. The whole assembly is then cured at elevated temperature. Due to variations in the thickness, or the weave structure of the reinforcements, or variations in the resin content, or variations in local temperature or pressure during the molding process, the surface quality or texture of the “bag-side” surface of the resulting parts may vary substantially. More importantly, the local thickness of the part may vary significantly, leading to variation in the surface contour of the laminate, especially in the bag-side surface. This causes the manufacturer to either accept loose tolerances for the dimensions of the assembled structure, or to incur considerable difficulty and increased expense in the assembly of such composite laminates with other components. Massive and stiff assembly jigs may be necessary to hold the components in position while shimming is carried out to compensate for the local irregularities of the surface contour of the components.
A process which could be used to produce or shape structural composite components, made using open molds, with accurate dimensions on any surface would be therefore be very desirable.
Current precision dimensioning processes include machining of the surface of laminates, where additional material is deliberately included in the original manufacture of the component and at least partly removed later in an additional machining process. Most machining processes involve micro-level fracture and frictional processes, and can generate significant amounts of heat. The heat generated can vary significantly within a short space of time, depending on the amount of material removed. This variable temperature can have an impact on the size of cutting tool and material under the tool, due to expansion that occurs with most materials upon application of heat, and therefore on the precision that can be achieved with a machining process. A second disadvantage with machining laminates to precise thicknesses is the difficulty in integrating a position sensor or thickness measurement system with a high speed moving component. Very high precision is generally achievable only when such sensors or measurement systems are used dynamically to control the position of the machining system, and locating the end of a high speed moving tool is difficult. It is more difficult still to pass a signal through a moving machining tool in order to effect precision measurements. Finally, if too much material is removed through the machining process, repair of the component involves a lengthy and expensive process of adding additional material, and in some cases the component is simply scrapped.
In some instances, particularly where the tolerance requirements for assembly are, for example, better than 50 μm, machining of surfaces may not be accurate enough for this requirement, and in practical terms such precision requirements are often met by using expensive and time-consuming shimming processes such as those outlined above.
The applicant has recently developed a process for shaping of thermoplastic surfaces on thermosetting composite laminates. This process is described in applicant's PCT application PCT WO 2005/025836 A1, the contents of which are incorporated herein by reference. This process has the key advantage that precision surface dimensions can be achieved on a thermosetting composite material following manufacture, either by using a quasi-static tool press or a moving tool. In the instance of a quasi-static tool used to create a precision component thickness, the thermoplastic surface is heated and the aforementioned tool pressed into the thermoplastic surface, imprinting it and allowing creation of a localized area of defined thickness. The accuracy of thickness is thus determined the accuracy of the tool, by the stiffness of the tool and pressure application system, the accuracy of tool stops or other means of preventing excessive impression of the tool, and the accuracy with which the thermosetting component is held down during this process. An additional limitation of the process utilizing a quasi-static tool is the questionable scalability of the process, as the achievable accuracy of the process may decrease with increasing tool size. The aforementioned recently developed process also includes the use of moving forming tools. These tools allow the scalable application of the process, however this technique is likely to have low levels of precision. In either case, achieving a surface profile accuracy greater than 100 μm for a practical component is unlikely.
Therefore, in order to utilize the invention described in PCT WO 2005/025836 A1 more fully and to achieve very high precision over large surface areas, it is necessary to improve the stated process. Furthermore, by providing an improved process and apparatus for surface re-profiling, the principle of providing a high accuracy surface can be extended beyond thermosetting composites with thermoplastic surfaces.